Alternating current (AC) power is typically supplied from wall outlets and is sometimes referred to as line power or mains power. Electronic devices often include circuitry that runs from direct current (DC) power. AC to DC power converter circuitry can be used to convert AC power to DC power. The DC power may be used to power an electronic device that runs on DC power. The DC power may also be used to charge a battery in an electronic device.
AC to DC power converters often include transformers. A transformer in an AC to DC power converter may have primary and secondary windings. A pulse width modulation (PWM) circuit on the primary side of a transformer may generate pulses of current that pass through the primary winding of the transformer. On the secondary side of the transformer, a diode may be used to rectify the output of the secondary winding.
Some AC to DC power converter circuits use synchronous rectifier (SR) output stages. SR output stages may include one or more metal-oxide-semiconductor field-effect transistors (MOSFETs). The MOSFETs are driven so as to rectify the output waveform from the transformer in the same way that the diode is used in other power converter designs, while avoiding high diode voltage drops when conducting current (e.g., approximately 0.7V).
Certain power converter designs may have potential drawbacks. One drawback is that different electronic devices may have different voltage requirements, and a single power converter may not be able to satisfy all of the requirements of all the different devices. For instance, a laptop computer may require a higher amount of power during operation than the amount of power required by a mobile phone, e.g., a laptop could require two to three times the amount of power (or more) of a mobile phone. The power converter used to operate the mobile phone may not provide a sufficient amount of voltage to operate the laptop and the power converter used to operate the laptop may provide too much voltage and overpower the mobile phone.
To attempt to deal with some of these drawbacks, some power converters may employ a “tapped winding” transformer configuration. A tapped winding transformer refers to a transformer where the turns-ratio of the transformer may be adjusted to produce high and low output voltage ranges, as desired. However, such configurations can result in an overly complex circuit design, e.g., when two or more voltage output levels are required. Other power converters may employ a “hybrid” or “cascaded” converter design which, for example, may be comprised of a flyback converter followed by a DC/DC converter, such as a “buck converter” circuit or “boost converter” circuit that is configured to use an inductor, capacitor, and one or more switching devices to decrease or increase, respectively, an input voltage to a different output voltage level. However, such configurations can result in voltage transitions that are not smooth and monotonous, but instead may have discernable discontinuities.